Apollo, in Greek mythology, is god of truth, the arts, archery, plague, divination and a few other things. Apollo is son of Zeus and Leto, and the twin of Artemis (goddess of the hunt). In later times he became equated with Helios, god of the sun, and by proxy his sister was equated with Selene, goddess of the moon. He is now primarily known as the god of the sun.
Apollo popularly (e.g., in literary criticism) represents harmony, order, and reason--characteristics contrasted by those of Dionysus, god of wine, who popularly represents emotion and chaos. The contrast between the roles of these gods is reflected in the adjectives Apollonian and Dionysian. However, Greeks thought of the two qualities as complementary: the two gods are brothers, and when Apollo at winter left for Hyperborea would leave the Delphi Oracle to Dionysus.